1. Field of the Invention
The present invention relates generally to methods for producing graphical images on computers and computer modeling. In particular, the present invention relates to a system and method for applying shading to a computer generated graphic image for more realistically modeling objects. Still more particularly, the present invention relates to the production and use of an accessibility factor for applying shading when rendering each pixel of an image.
2. Description of the Related Art
Computer graphics and systems have become well known. Every image generated by such computer systems comprises thousands of pixels. Each pixel is selectively illuminated with various degrees of intensity and color to produce graphical images. The ability of computers to generate such images has become useful in a variety of applications and computer modeling techniques. One continuing problem in the area of computer graphics has been producing an image of an object that matches the item's appearance in the real world. Often computer generated images are readily recognized as such because they do not realistically depict the real world object. Typically, objects that are modeled by computers have a new, clean and sterile appearance that lacks surface texture or other effects that come with the aging of an object. It continues to be very difficult to model objects with an aged appearance.
One prior art approach to eliminate this problem and to provide computer graphics with more flexibility for representing objects has been the application of surface texture and shadowing. Texture has been applied to surfaces in a number of ways. For example, surface texture has been applied using parametric surface textures, using a pre-filtering pyramid for anti-aliasing, and using solid textures based on the position of a surface element. Various illumination methods have also been used on images to help obtain realistic soft shadows. However, these prior art approaches are often rendered such that they are dependent on the view of the object being displayed and the positioning of light sources. This is problematic because the surface texture and shading are continually being reproduced as views of the model changes, and each production of surface texture and shadowing requires a significant amount of computational resources.
Another prior art approach is disclosed by Saito and Takahashi in "Comprehensible Rendering of 3-D Shapes," Computer Graphics, August 1990, pp. 197-206. Saito and Takahashi disclose a process of finding lines of slope discontinuities that are a single pixel wide. These lines are then used to enhance the rendering of the image. While this method is useful for intelligible rendering, it cannot be used to provide a smoothly varying estimate of the accessibility of the surface regions of an object. Moreover, like the other prior art solutions, the lines must be recalculated for each new view of a scene. The line discontinuities do not have a global application that can be used for all shading.
Other prior art approaches to realistic rendering are disclosed in the context of molecular modeling. For example, a solvent molecule has been described as a sphere of some fixed radius and then the solvent-accessible surface defined as the boundary of the volume that could be occupied by the sphere without penetrating the molecule. In the case of a molecular model made just of spheres, the solvent accessible surface is made up of pieces of spheres and tori. (See Lee and Richards, "The Interpretation Of Protein Structures: Estimation Of Static Accessibility," Journal of Molecular Biology, 1977, p. 151.) Another prior art approach provides a measure of surface convexity. It is computed by placing a sphere center on the molecular surface point, and then computing the fraction of the sphere's area which is contained within the molecular surface. (See Connolly, "Measurement Of Protein Surface Shape By Solid Angles," Journal of Molecular Graphics, 1986.) Yet another prior art approach defines surface accessibility as the radius of the largest sphere which may touch the surface tangentially and not intersect other surfaces. (See Kuhn et al., "The Interdependence Of Protein Surface Topography And Bound Water Molecules Revealed By Surface Accessibility And Fractal Density Measures," Journal of Molecular Biology, 1992, pp. 13-22.) While these methods may be used to provide more realistic rendering of images, they are still problematic in several respects. Because the methods were originated in the context of molecular modeling, they provide accessibility of only a sphere in the presence of another sphere, and therefore, do not provide a basis for effectively modeling all shape types and determining the accessibility for each. Moreover, determining accessibility as defined by the prior art is also very taxing on computer resources. Accessibility computations require large amounts of processing power and can adversely effect the overall performance of the system for other operations. Thus, there is a need for a method of determining accessibility that does not adversely affect computer performance.
Therefore, there is a need for a system and method for effectively producing images that realistically show aging, in particular, the accumulation of dirt and other foreign matter upon objects. Moreover, there is a need for a system and method that efficiently produce and render such images on a display device.